Unstable Flow Structures Present at Different Rotational Velocities of the Centrifugal Compressor

Abstract

Unstable flow structures cause inevitable energy losses in all power energy systems, including turbomachines. In this study, a set of analyses was conducted with the use of spectral maps on the pressure signals obtained from an industrial centrifugal compressor. The spectral maps provide one a detailed visualization of the flow conditions present in the machine along the performance curve and to distinguish the flow phenomena present prior to the surge. The method accuracy is especially useful in detecting the inlet recirculation. The study was conducted at four impeller rotational speeds with varying loads imposed by a valve at the outlet. At each speed, the machine experienced different stages of unstable flow conditions prior to the surge. Five main frequency peaks that appeared in all cases were identified and discussed. The surge was observed at all impeller speeds. At lower ones, however, it appeared at higher valve closures. At higher speeds, the surge was much more intense. The study has also shown that the inlet recirculation appears also for the closed-type industrial impeller. The phenomenon was present in all conditions. The higher impeller speed, the faster onset of the inlet recirculation was. This structure has a strong potential for an early instability warning because it appears in various types of impellers, has a very particular spectral structure and its positioning is very predictable. This study gives another example of the inlet recirculation universality and potential for efficient anti-surge protection.

Aerodynamic instabilities detection via empirical mode decomposition in centrifugal compressors

“…It means that the intensity of the inlet recirculation is changing in time for the same operating conditions, as some windows register stronger and some weaker signature of this instability. This is in line with other studies showing that inlet recirculation has a non-periodic character and its intensity may vary in time [45].…”

Singular spectrum analysis as a tool for early detection of centrifugal compressor flow instability

“…It is known that the spectral structure of certain instabilities are influenced by this parameter. It has to be pointed out, however, that recent studies have shown that the inlet recirculation onset point also changes with the impeller speed and always precedes the surge by a similar distance along the performance curve [52]. This means that the protection & control systems based on inlet recirculation are indeed reacting prior to the surge for a wide range of the speed lines.…”

An unsteady flow control technique based on negative circulation conception and its application to a blade-divergent passage

“…The optimization design of the blade profile is the main way to improve the flow angle, but the accompanying large diffuser makes it easy to cause flow separation in the blade passage, which makes it difficult to significantly improve the performance of the compressor further. Among these flow separations, the blade suction surface separation flow is one of the most typical complex separation flows in high-load compressors, which will lead to negative factors such as increasing losses, passage blockage, reducing efficiency, and even inducing stall or surge, which will seriously affect the stable operating range of the compressor (Gbadebo et al, 2005;Lei, 2006;Choi et al, 2008;Likiewicz et al, 2020). Therefore, based on the in-depth understanding of the separation flow mechanism of the blade suction surface, it is necessary to introduce an effective flow control technology to suppress or weaken the flow separation in the compressor so that the compressor can maintain a high load while having a wide stable working range and efficiency, which is one of the key research directions in the compressor field at present.…”